In the past, gear pumps employing a meshed gear set have been used to draw fluid from an input or suction port within a pump housing and to pressurize and pass the fluid to an opposed output or pressure port within the pump housing. Conventionally, such gear pumps have included two elongate meshed gears extending longitudinally of the pump housing between the suction and pressure ports which are located on opposite sides of the meshed gears. The gears are mounted to rotate in gear pockets in the pump housing, and hypothetically when rotating seal against each other in the areas where the gear teeth mesh so that fluid from the suction port is carried around the perimeter of a gear pocket into the pressure port. This action pressurizes the fluid being delivered to the pressure port, and the resulting pressure gradient between the pressure and suction ports results in fluid leakage through any clearances present between the teeth of the meshing gears. These clearances invariably exist due to gear tooth lead error which is waviness or profile error of the involute along the length of the gear. Lead tooth error provides a fluid flow path which increases in area as the axial length of the gears increases, thereby resulting in degradation of the volumetric efficiency of gear pumps employing only two meshed gears.
The volumetric efficiency of known gear pumps is further degraded by fluid leakage between the suction and pressure ports around the ends of the gears. Thus gear tooth manufacturing lead error and the gear end clearance relative to the gear housing result in significant internal fluid pumping losses for a gear pump.
Gear pumps have often been employed as fuel pumps for internal combustion engines, and to meet demands for ever increasing fuel system efficiency, engine performance and lower emissions, it has become necessary to enhance the volumetric efficiency of gear type fuel pumps. To accomplish this, fluid leakage between the low pressure and high pressure portions of the pump must be minimized.